Effects of Carbon Functionalization on Pd/C Electrocatalysts for Alcohol Oxidation and Alkaline Direct Alcohol Fuel Cells

Abstract

This study examines how KOH and HNO3 pretreatment of Vulcan XC-72 changes the physicochemical and electrochemical behavior of Pd/C catalysts for CO, ethanol, and glycerol oxidation in alkaline media. Pd nanoparticles were deposited by borohydride reduction. The treatments increased the BET surface area but decreased the measured Pd content, while altering oxygen functionalities, the Pd0/Pd2+ surface distribution, and the particle-size distribution. Electrochemical results showed that these structural and surface changes were associated with differences in the electrochemical response of the catalysts rather than being governed solely by electrochemically accessible surface area. Pd supported on KOH-treated carbon (Pd/CKOH) showed the lowest electrochemically active surface area but the highest ECSA-normalized peak current densities for ethanol and glycerol oxidation, whereas Pd/C HNO3 showed the lowest ethanol oxidation onset potential. Relative to Pd/C, the CO oxidation onset shifted negatively by 70 mV for Pd/CKOH and 40 mV for Pd/C HNO3, accompanied by treatment-dependent differences in the CO-stripping profiles. The alcohol oxidation onset potentials and peak current-density rankings depended on the alcohol and did not follow a single catalyst descriptor. Single-cell alkaline direct alcohol fuel cell tests further showed that normalization by the actual Pd loading affected the catalyst comparison. On a Pd-mass basis, Pd/CKOH exhibited the highest peak current and power densities for both ethanol and glycerol at all tested temperatures. Overall, the results indicate that support pretreatment is associated with concurrent changes in support surface chemistry, Pd surface chemical state, crystallographic texture, and electrochemical response, which together accompany the observed differences in electrocatalytic performance. These findings highlight the importance of considering multiple physicochemical and electrochemical descriptors, as well as catalyst loading and operating conditions, when evaluating the effects of carbon-support functionalization on Pd-based electrocatalysts.